Marine capsizings are one of the major causes of loss of life and property at sea. Capsizing occurs when a boat loses stability, i.e., when it loses the ability to right itself after it has been to one side by the forces of wind or water. An accepted measure of a vessel's stability is its GM value, defined as the distance between the vessel's transverse metacenter and its center of gravity. The transverse metacenter of a ship can be thought of as the maximum height to which the center of gravity can rise before the ship loses its initial stability. The position of a ship's center of gravity depends upon a number of time varying factors such as cargo loading, movement or shifting of cargo, fuel consumption, icing, and absorption of water by deck cargo. As the position of center of gravity changes, so does the vessel's GM value and its stability. If the center of gravity rises to a point above the transverse metacenter, GM becomes negative and the ship will not return to even keel and is in danger of capsizing.
When a vessel's transverse metacenter is known, it is possible to compute GM by careful consideration of the weight and distribution of fuel and cargo. Analog and digital computing devices have been developed in an effort to automate such a calculation. While of considerable value, static calculations of this kind do not take into account factors such as icing and shifting of cargo which can rapidly change GM and threaten the vessel's stability.
An instrument capable of continuously monitoring GM is described in U.S. Pat. No. 1,860,345. The apparatus shown in that patent determines GM from a vessel's maximum roll angle in conjunction with its maximum rate of roll. The former is measured by a pendulum, and the latter by determining the maximum force on the bearings of a gyroscope. The device is mechanically quite complex, and would require careful calibration to perform properly. Furthermore, the technique disclosed in that patent requires knowledge of the radius of gyration, a quantity not readily available for many vessels.